1. Field of the Invention
The present invention relates to an exposure control device for a camera having a photographic lens of variable focal length type which includes means for detecting a luminance of an object to be photographed and means for automatically controlling an exposure in accordance with the luminnnce detected by said detecting means.
2. Description of the Related Art
In general, in a camera having a photographic lens of variable focal length type, an open F-number of the photographic lens changes, depending upon a variation of the focal length. Accordingly, in case of a camera having an automatic exposure control device of the type in which a luminance of an object to be photographed is directly measured on the basis of a light which has not passed through a photograhic lens, it is necessary to correct the automatic exposure control device, depending upon the variation of the focal length of the photographic lens.
Heretofore it has been proposed to provide a correcting system for correcting the automatic exposure control device in which a light guide, a shielding slit or a filter is disposed at the front of a light measuring element to increase or decrease a quantity of light incident thereon, thereby effecting the correction of the automatic exposure control in response to the change of the open F-number of the photographic lens caused by the variation of the focal length thereof. (For example, see U.S. Pat. No. 4,451,129).
In such correcting system, however, the provision of the light guide, the shielding slit or the filter, which is required to be driven into or out of an optical path of a light measuring element, causes such disadvantage that the light guide, the shielding slit or the filter used is very complicated in shape and a driving mechanism or other accessory mechanism required therefor is also very complicated, so that a cost of a product is increased, a very high precision is required and a large space is required.
It has been also proposed to provide a correcting system including a movable member which moves depending upon the variation of the focal length of the photographic lens, an electric switch arranged to be turned on or off by the movement of said movable member and an electric circuit which receives an electric signal from the electric switch to correct the automatic exposure control device. (For example, see Japanese Laid-Open Patent Application No. Sho 58-182623.)
This correcting system, however, has such disadvantage that when it is applied to a lens, such as a zoom lens, in which its open F-number makes continuous change, it is required to divide the changing range into a number of sections as required to obtain a desired precision of correction and to effect corrections of respective ones of the representative open F-numbers.
In this case, the number of the switches (number of contact pieces) is increased, so that wiring of the number of switches causes many troubles in view of their number as well as volume and further provides problems in cost, reliability and precision.